Recently, there has been developed a white LED device with a yellow phosphor such as YAG being disposed in the vicinity of a gallium nitride (GaN)-based blue LED (Light-Emitting Diode) element. In this white LED device, blue light emitted by the blue LED element and yellow light emanated by the phosphor upon receipt of blue light are mixed to obtain white light. Further, there has also been developed a white LED device in which blue light emitted by a blue LED element and red light, green light, or the like emanated by a phosphor upon receipt of blue light are mixed to obtain white light.
In such a white LED device, there is disposed an encapsulating member in which phosphor particles are dispersed in a transparent resin. The specific gravity of phosphor particles contained in the encapsulating member is greater than the specific gravity of the transparent resin. Therefore, there has been a problem in which, at the time of the formation of the encapsulating member, phosphor particles easily settle before curing of the transparent resin, causing the concentration of the phosphor not to be uniform inside the encapsulating member. If the concentration of the phosphor is not uniform, the chromaticity of irradiation light of an LED device is hard to fall within a desired range.
Under such circumstances, it is proposed that, for example, a silicone resin with high viscosity is adopted as an encapsulating resin, making it difficult for phosphor particles to settle (PTL 1). Further, it is also proposed that settling of the phosphor particles is suppressed by: adding a laminar clay compound and an organic cation to an encapsulating resin (PTL 2); and by adding silicone microparticles to an encapsulating resin (PTL 3).
According to the techniques of PTLs 1 to 3, the setting and segregation of phosphor particles can be suppressed to some degree. However, any of the techniques of PTLs 1 to 3 ends up encapsulating phosphor particles with resin. Therefore, there have been a problem in which light and heat from an LED element cause an encapsulating resin to be easily colored and deteriorated.
For these problems, there is proposed a technique in which a composition containing phosphor particles and a ceramic precursor is applied onto an LED element to bind phosphor particles with a ceramic binder. The ceramic binder is stable to light and heat from an LED element. Therefore, coloring or deterioration over time is rare. However, in the composition in which phosphor particles and a ceramic precursor are mixed, there is a case where, if it is stored for a long period of time, the phosphor particles and the ceramic precursor react with each other, causing the ceramic precursor to be gelated, solidified, or the like. Under such circumstances, it is proposed that phosphor particles and a ceramic precursor are applied separately (PTL 4). According to the method described in this literature, 1) a phosphor dispersion liquid containing phosphor particles is applied onto an LED element to form a phosphor particle layer; and thereafter 2) a solution containing a ceramic precursor is applied onto the phosphor particle layer to bind the phosphor particle layer with a translucent ceramic binder. According to this method, there is less risk that a phosphor dispersion liquid or a solution containing a ceramic precursor may be gelated during storage.